Origin of the Normal Electrocardiogram

Question 1

label the x and y axes of the ECG

Answer 1

y axis is voltage in mV
x axis is time in seconds

Question 2

describe cell surface events when an upward or downward deflection is recorded on the ECG

Answer 2

1. if a wavefront of negative charges (wave of depolarization) moves toward a positive electrode, an upward/positive deflection is recorded
2. is a positive electrode detects a positive membrane potential relative to that detected by the negative electrode, the ECG will show a downward deflection
3. if a wave of negative charges moves toward the negative electrode, ECG records a downward deflection

IMPORTANT: ECG records extracellular events, AP represents transmembrane events, so a wavefront of negative charges or depolarization refers to charge or outside of cell

Question 3

name 3 major factors that determine the size of the ECG recording

Answer 3

1. strength of voltage source (m): mass of muscle depolarized; bigger ECG with greater mass (like ventricles)
2. distance (r) between electrodes and wavefront: bigger when closer but not clinically relevant
3. angle (theta) between axis of recording lead (pair of electrode) and wavefront dipole: recording lead only see the component of the wavefront vector that is parallel to the lead axis, so the biggest deflection is recorded when the lead axis and wavefront are perfectly parallel and no deflection is recorded when the lead axis and wavefront are perfectly perpendicular

Question 4

when given a normal ECG label the individual waves and explain the cellular events that each represents

Answer 4

P wave: atrial depolarization
QRS complex: ventricular depolarization
Q wave: left to right depolarization of interventricular septum (away from positive electrode; negative deflection)
R wave: endocardial to epicardial depolarization of the ventricles (sum vector is cranial to caudal, right to left) (toward positive electrode; upward deflection)
S wave: upward depolarization of base of ventricles (away from positive electrode = negative deflection)
ST segment: isoelectric baseline connecting S and T waves where all cells are depolarized and in the plateau phase of the action potential
T wave: ventricular repolarization

Question 5

when provided information about the pattern of cardiac depolarization, predict the ECG’s appearance